Because the emissions from natural gas are about 45 percent lower per Btu than coal, some have suggested that newly revised estimates of U.S. shale gas reserves could mean that gas will be a “bridge” to a lowcarbon future. Research from RFF’s Center for Energy Economics and Policy indicates that while natural gas may substitute for coal, it won’t substitute for a carbon policy. The reason: gas also competes with low-carbon energy sources, like renewables and nuclear power. Without a carbon policy, the greenhouse gas emissions from greater use of natural gas are modest, although new natural gas supplies will lower the cost of climate policy. So natural gas is a bridge, but a flimsy one.